TGI Fridays faced protests outside restaurants in London and Milton Keynes as waiting staff joined in the UK’s first strike over tipping.

In February, the casual dining chain, which has 83 outlets, began redistributing 40% of service charge payments paid on credit and debit cards to back-of-house employees, including kitchen staff, in lieu of a wage increase.

Workers say the change is linked to increases in the legal minimum wage, which has forced the restaurant to put up waiters’ pay. Those increases, they say, have gradually eroded the differential with kitchen staff who have historically earned a bigger salary but not had a share of tips.

Speaking outside TGI Fridays in Covent Garden, central London, where strikers and supporters were chanting: “Forget the burgers, forget the chips, we want fair tips”, one waitress who has worked for the restaurant for more than a decade told the Guardian: “I absolutely think the kitchen staff should be paid more. I just don’t understand why I am paying them.”

The waitress said the change in tips policy meant a minimum cut in pay of 10% to 15% for waiting staff who had already seen a fall in tips as more people pay by contactless or skip tipping. She calculated that she had lost about £250 since the end of February and had gone overdrawn for the first time since leaving university.

The waitress added that the cut in tips was only the latest in a series of measures that had trimmed waiters’ benefits. “Every time the minimum wage has gone up they have taken something back. We used to get paid extra on bank holidays, now we don’t. Shift leaders used to get extra pay, that’s gone.”

Two restaurants, in Covent Garden and Milton Keynes, were on strike on Friday while two more in the Trafford Centre, Greater Manchester, and Piccadilly, London, have voted to join action set for next Friday.

Dave Turnbull, the regional officer for Unite, which has been supporting the workers, said: “We need the company to wake up and talk to us. The only way for this to be resolved is to sit down and talk about a way forward.”

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A TGI Fridays spokesperson said: “Our team members are a part of our Fridays family and we care about them. We believe all our team members should be – and are – treated and paid fairly … We are listening [to the workers] and are collectively working to find a resolution. In the meantime, we will be doing all we can to ensure our guests receive the usual great dining experience and that the restaurant remains open as normal.”

The TGI’s dispute comes amid tough times for casual dining chains struggling with rising costs, more competition and a squeeze on customer spending. Jamie Oliver’s Italian chain, Prezzo and Byron have all closed multiple outlets and asked their landlords for rent cuts.

This week, Carluccio’s, the Italian-inspired chain, has admitted it may close 30 outlets. Carluccio’s is also seeking 67% rent cuts at about a third of its 103 sites for six months while it considers whether it can keep them open.